1. Field of the Invention
The present invention relates to an improved apparatus and method for the production of alloys by a physical vapour deposition process generally as described in our earlier British Patents GB 1206586 and GB 1265965. In such a process, the apparatus is operated under vacuum within a vacuum chamber and the alloy constituents are evaporated from one or more evaporation baths before being caused to condense upon a temperature-controlled collector.
The apparatus and method described here are suitable for producing alloys in substantial quantities and with sufficient structural integrity that the deposits may be removed from the collector intact. The deposit can then be worked into sheet, strip or other wrought form and heat-treated to achieve the desired mechanical properties. As with conventionally-cast alloys, the deposited alloy may be heat-treated before, during or after working. Alternatively, the deposited alloy may be removed and pulverized for subsequent powder metallurgical techniques, for example when it is desired to produce an article close in form to the intended final shape.
2. Discussion of Prior Art
Recently, interest has grown in the possibility of obtaining improved magnesium alloys by creating new compositions using rapid solidification rate (hereinafter referred to as RSR) production processes. Although magnesium is the lightest of the structural metals, its alloys have yet to find widespread use in aerospace applications, partly because of certain shortcomings in their mechanical properties, but principally because of their poor corrosion resistance. In magnesium alloys produced by conventional non-RSR methods the addition of elements such as aluminium, chromium or silicon is ineffective in improving the corrosion resistance even though such additives are known to form protective surface films in other alloy systems. This inefficacy is due to their poor solubilities in the magnesium matrix: Under normal equilibrium conditions the concentration of such additives in solid solution is tool low to provide an effective barrier to corrosion.
In magnesium alloys it is generally believed that corrosion-inhibiting additives should be incorporated in the form of solid solutions so that a uniform electrode potential is maintained throughout the alloy. If the additives are allowed to segregate and form precipitates, and if the precipitates have a different electrode potential from the matrix, then they effectively behave as tiny Galvanic cells and corrosion resistance is worsened rather than improved.
Rapid solidification techniques including vapour deposition provide the means to overcome equilibrium thermodynamic constraints and achieve compositions which are beyond the scope of the ingot metallurgist by "freezing" constituent atoms in position before they have the opportunity to migrate and segregate as they would in conventional ingot processes. These techniques therefore offer a possible route to improved corrosion resistance in magnesium alloys by allowing an increase in the population of corrosion-inhibiting species in the magnesium alloy without forming harmful precipitates.
Physical vapour deposition is favoured over other forms of RSR processing for a number of reasons. Firstly, the cooling rate is very much higher in physical vapour deposition and thereby increases the likelihood of formation of solid solutions. Secondly, physical vapour deposition offers a considerable choice of potential alloying constituents since the candidate elements are raised to the vapour state, thereby ensuring miscibility between different constituents. By contrast, other forms of RSR processing are limited in the possible combinations of alloying constituents which they can offer to those elements which are miscible in the molten state. This is a particularly important consideration in the case of magnesium since, at the melting temperature of many potentially interesting alloying additions, magnesium has a very high vapour pressure and hence evaporates very quickly.
The use of physical vapour deposition to prepare magnesium alloys containing additives such as aluminium, chromium or silicon is not without its problems, however, owing to the large differences between the vapour pressure of magnesium and the vapour pressures of the additives. Where uniform composition of the deposit is a prime requirement, it is preferable to use a single evaporation source, but when the alloy constituents have vapour pressured differing by several orders of magnitude, as in the magnesium-chromium system for example, single source evaporation is no longer practicable. In these circumstances, separate sources are required and the design of the apparatus becomes more complicated. If separate sources are arranged side-by-side the composition of the deposit is non-uniform across the substrate due to imperfect mixing of the vapour streams. Better mixing can be achieved by increasing the separation of the collector from the sources but this has the effect of lowering the deposition rate.
Another way of minimising non-uniformity is to introduce lateral movement between the relative positions of the collector and the sources. In practice, it is easier to keep the sources stationary and to move the collector, either by rotation or by a reciprocating motion, thereby ensuring that exposure of different parts of the substrate to the respective sources is equalized. Whist a moving collector offers considerable benefit in improving the homogeneity of the deposit, a small degree of non-uniformity is inevitable because the deposit is effectively laid down as a series of sub-layers which are alternatively rich in one particular constituent. In a structural member even this level of non-uniformity could be critical to its overall strength and might also be bad from the point of view of corrosion resistance.
In an earlier British Patent Application number GB 2 230 792 A we describe a method suitable for producing magnesium alloys by physical vapour deposition which uses separate evaporation sources mounted on a vertical axis wherein the sources are arranged to discharge their respective vapours into a heated chimney. The chimney is maintained at a temperature which is at least as high as the temperature of the hottest evaporation source in order to suppress condensation of vapour on the chimney walls. Vapours impinging on the chimney walls are therefore recited, thus promoting vapour mixing. Use of this method is limited to those elements which can be easily evaporated using radiant heating, since the tantalum heaters used to heat the highest temperature evaporation source are rapidly degraded if the temperature of this source exceeds 1450.degree. C. As long as tantalum remains the material of choice for such heaters then the operating temperature range will be limited to this ceiling, so for the evaporation of certain potentially useful alloying additions other methods must be devised.